The use of asbestos as a diaphragm material in electrolytic chlor-alkali cells is well known. Ordinarily the diaphragms are prepared by vacuum-drawing a slurry of asbestos fibers onto a porous cathode.
It has been previously taught that polymeric fluorocarbons (also known as fluoropolymers) may be used as binders for asbestos diaphragms. The technique involved is, in general, the mixing of particulate polymer binder material with the slurry of asbestos, then the drawing or depositing of the slurried materials in the form of a matte on the porous cathode, then heat-sintering to effect bonding. The fluoropolymers generally reduce the wettability of the diaphragms.
Of particular relevancy to the present invention is the fluoropolymer-bonded mixture of chrysotile asbestos and crocidolite asbestos disclosed in U.S. Pat. Nos. 4,093,533. These fluoropolymer-bonded mixtures of asbestos are suitable for use as membrane material in electrolytic chlor-alkali cells, withstanding even the harsh attack of highly acid electrolytes. However, there is a need to improve the wettability (hydroplilicity) of such diaphragms. Other patents relating to fluoropolymer-bonded asbestos diaphragms are referred to in U.S. Pat. No. 4,093,533.
The use of anolyte (NaCl brine) or catholyte (NaOH solution) as the aqueous medium for the slurry of asbestos and fluoropolymer have drawbacks. Any NaCl in the drawn diaphragm tends to cause accelerated rusting (chloride attack) of the foraminous steel cathode onto which the diaphragm is drawn and then heat-bonded. Any NaOH in the drawn diaphragm can cause caustic burns to workers performing the diaphragm-drawing and baking and it represents a safety hazard.
It is an object of the present invention to improve the hydrophilicity (wettability) of fluoropolymer-bonded asbestos diaphragms.
Another object is to provide an ingredient in a fluoropolymer-bonded asbestos diaphragm which not only improves the hydrophilicity of the diaphragm, but which is innocuous to persons preparing the diaphragms and which does not attack the ferrous cathode substrate.